In recent years, infrared imaging devices receiving infrared light for imaging have been widely used. For example, a night vision system is commercially practical, which includes an infrared imaging apparatus mounted on a vehicle for capturing images of the surroundings of the vehicle during the night, and when detecting a pedestrian who may collide with the vehicle, issues a warning to the driver. In order to capture an image, the infrared imaging apparatus includes an infrared lens passing and collecting infrared light and an infrared image pickup device receiving the infrared light.
In such an imaging apparatus, in order to facilitate production or assembly, one or more lenses for imaging are held in a cylindrical barrel to form a unit. FIG. 27 shows a schematic view showing a configuration of a conventional vehicle-mounted infrared lens unit 104. Vehicle-mounted infrared lens unit 104 includes a cylindrical barrel 130 in which a first infrared lens 110 and a second infrared lens 120 are held inside. Infrared lens unit 104 is configured such that first infrared lens 110, a spacer 140, and second infrared lens 120 are inserted in order in barrel 130 and are fixed by a lens retainer 150 in a state in which they are fitted in the inside of barrel 130. In the following, the second infrared lens 120 side in the axial direction is referred to as the front side, and the first infrared lens 110 side is referred to as the rear side.
Barrel 130 is provided with a lens holding portion 131 formed like a step around the inner circumferential surface on the rear side. First infrared lens 110 inserted in barrel 130 is fitted in the inside of lens holding portion 131 and thus hold. Spacer 140 is shaped like a cylinder having an outer diameter approximately equal to the inner diameter of barrel 130 and is inserted and fitted in the inside of barrel 130. Second infrared lens 120 has an outer diameter approximately equal to the inner diameter of barrel 130 and is inserted and fitted in the inside of barrel 130 in the vicinity of the front-side end thereof. First infrared lens 110 and second infrared lens 120 inserted and fitted in barrel 130 are fixed to barrel 130 using adhesive or the like after being subjected to positional adjustment (alignment) of the optical axis.
In barrel 130, the end surface of the rear side of spacer 140 is in abutment with a peripheral portion of the front side of first infrared lens 110, and the end surface of the front side of spacer 140 is in abutment with a peripheral portion of the rear side of second infrared lens 120. In other words, spacer 140 is a member sandwiched between first infrared lens 110 and second infrared lens 120 in barrel 130 for positioning those infrared lenses with respect to the axial direction. The distance L1 between those infrared lenses is defined by the axial length of spacer 140.
Since the temperature in the surrounding environment around the infrared imaging apparatus mounted on a vehicle varies widely, a change in optical characteristics of vehicle-mounted infrared lens unit 104 due to a temperature change becomes a problem. For example, when the refractive indexes of first infrared lens 110 and second infrared lens 120 of vehicle-mounted infrared lens unit 104 are varied with a temperature change, a focal shift with respect to the image pickup device occurs. As a result, for example, an image captured by the image pickup device is blurred, and the performance of the infrared imaging apparatus is degraded. A technique for avoiding this problem is sought.
Patent Document 1 proposes an optical apparatus including a substrate on which a functional element such as a CCD (Charge Coupled Device) is attached, a tubular holder having one end fixed to the substrate, and a tubular barrel having one end attached to a lens, wherein the barrel is arranged along a direction approximately vertical to the light-receiving surface of the functional element, and the other end of the barrel is attached to the other end of the holder. In this optical apparatus, even when the temperature of the environment during use changes and the holder is thermally expanded/shrunken with respect to the substrate, the barrel is thermally expanded/shrunken in the direction opposite to that of the thermal expansion/shrinkage of the holder, thereby preventing a focal shift of the lens.
Patent Document 2 proposes an image scanning apparatus including a carriage capable of reciprocating parallel to a platen on which a document is placed, wherein an imaging position can be maintained on a line sensor surface of the carriage even when the usage environment changes. In the carriage of this image scanning apparatus, a chip member for fixing a lens barrel and a housing accommodating the lens barrel has one end adhered to the lens barrel by adhesive and has the other end fixed to the housing by a screw. The thermal expansion coefficient of the chip member is set greater than the thermal expansion coefficient of the housing, so that the optical path length from the document to the collective lens and the optical path length from the collective lens to the line sensor can be changed even when the housing is thermally expanded or thermally shrunken due to a temperature change of the usage environment. The imaging position can thus be maintained.